Retroreflectivity is a phenomenon whereby significant amounts of light irradiating an object are returned back in the direction from which the light comes. This can be achieved on, for example, a traffic marking by using appropriate inorganic materials in particulate form, such as glass beads on the surface of the material. The glass beads are especially advantageous because they act as lenses focusing light onto pigment in the traffic marking and then redirecting the light back to the source.
Inorganic materials, especially vitreous beads, are widely used as additives in polymeric compositions, such as those deposited on highways, to provide retroreflective markers, for instance, edge and lane striping, signs, etc. As is well known, it has become common practice to drop small glass spheres onto a painted line on a highway while the paint is still wet, or at least tacky, such that the spheres are partially embedded in the paint when it has dried. The spheres render the line or other marker retroreflective, and return light from headlights so that the marker is more visible to the motorist. In some cases the spheres are embedded in spherical or irregularly shaped plastic granules prior to being deposited on the paint in the manner disclosed in U.S. Pat. Nos. 3,252,376 and 3,254,563, for example, in an effort further to improve the reflectivity of the marker.
It is known that glass particles greatly increase the visibility of painted markings on roadway surfaces and other painted surfaces when the glass particles are dispersed into the painted surface. Optimum embedment of a glass bead for retroreflectivity and durability is typically 60% of the bead diameter. Less than this can affect durability adversely and more than 60% reduces the retroreflectivity. A surface treatment that "floats" a bead at 60% of its diameter can thus optimize the simultaneous attainment of retroreflective performance and embedment-durability.
A problem encountered with retroreflective particles is an inability of the particles firmly to bind with the paints or other materials into which they are placed. Thus, increased visibility imparted by glass particles can be short-lived due to a steady loss of the glass particles as they are loosened and removed by friction, weather changes and other physical factors.
The loss of glass particles can be slowed by coating the particles with thin layers of certain coupling agents selected for their ability to provide positive adhesion between the particles and surrounding materials. However, utilization of such agents typically entails its own set of disadvantages. Known coupling agents often reduce the retroreflectivity of glass particles by causing a process known as "wicking", whereby paints are drawn onto the glass surface and coat too much of the surface area for sufficient retroreflectivity to be achieved. Although normal road abrasion wears away some of the paint coverage which results from wicking, retroreflectivity never becomes ideal, and in any event is diminished for an undesirably long period of time after the glass is placed. Furthermore, evenly dispersing glass particles into a painted surface can be difficult unless the particles are free-flowing, but known coupling agents often are not hydrophobic and particles coated therewith are subject to moisture-induced agglomeration. Because the particles used are typically so small as to have a powdery appearance, small amounts of moisture can cause the particles to agglomerate and lose important free-flowing properties. Even high relative humidity or condensation on the particle surfaces caused by temperature changes can provide sufficient moisture to cause agglomeration and to interfere with flow properties. Accordingly, this technique for enhancing adherence is frequently problematic.
Another technique for improving retroreflectivity is to blend different glass particles, of the same or different sizes and having coatings conferring different and desired properties or no coating at all. In particular, it would be advantageous to deposit on the paint a mixture of (i) beads coated with a component for facilitating flotation of the beads in the paint, (ii) beads coated with a component for facilitating adherence of the beads to the paint, and (iii) uncoated beads. In this manner, the beads would have a layered distribution in the paint. In principle, the uppermost layer would be the beads coated for flotation, the middle layer would be the uncoated beads and the bottom layer would be the beads coated for adherence. As the paint wears down by friction, weather changes or other physical factors, each layer in turn would be exposed to provide retroreflectivity. Flotation properties are imparted by certain fluorochemicals, which by their insolubility in common solvents in turn impart flotation properties in many paints containing organic solvents or solvent-like vehicles. Unfortunately, these fluorocarbons do not adhere to glass with permanence and migrate onto all available surfaces, including uncoated glass beads. Therefore, this ideal layering cannot be achieved in practice since conventional flotation-coatings migrate to all of the beads in the blend so that all of the beads act as flotation-coated beads.
It would be a substantial advance to provide a particle comprising inorganic material which is both retroreflective and adherent to the polymeric composition in which the filler material is embedded, and the flotation-inducing coating of which does not exhibit undue migration when in admixture with other particles. Moreover, it would be most desirable to increase adhesion of the bead to the paint when flotation-coated beads are used because flotation coatings severely reduce adhesion of glass to paint.